As disclosed in, for example, JP-A-2007-37376, a switching power supply generally has an overcurrent protection function to protect circuit devices from overcurrent caused by various factures. Specifically, the overcurrent protection function detects electric current flowing through a switching device and forces the switching device to be turned OFF when the detected current value exceeds a predetermined overcurrent threshold.
In this case, it is preferable that a detection error due to inrush current or noise at the time of power-on should be prevented as far as possible for the following reasons. An output stage of a switching power supply is usually connected to another power supply such as a series-regulator power supply, and an output of the other power supply is supplied to a control system such as a microcomputer. In this structure, if the overcurrent protection function is frequently activated due to a detection error caused by noise or the like, the switching device is also frequently forced to be turned OFF, so that the other power supply may be frequently stopped accordingly. As a result, the control system may stop operating.
Such a problem may be overcome by setting the overcurrent threshold to a relatively high value or by adding an overheat protection function instead of or in addition to the overcurrent protection function. The overheat protection function detects a temperature of the switching device and forces the switching device to be turned OFF when the detected temperature value exceeds a predetermined overheat threshold. However, these approaches may have the following disadvantages.
When the overcurrent threshold is set to a relatively high value, the circuit devices (e.g., diodes, inductors, capacitors, etc.) of the switching power supply need to have a current capacity higher than a current capacity required to meet specifications. Therefore, setting the overcurrent threshold to a high value unnecessarily results in an increase in size and cost of the switching power supply.
The overheat protection function can be added if the switching device and a control circuit for controlling the switching device are incorporated in a single semiconductor integrated circuit (IC). In contrast, if the switching device is added as an external device to the IC which incorporates the control circuit, it is difficult or impossible to add the overheat protection function for the following reasons.
When the switching device is incorporated in the IC together with the control circuit, characteristics of the switching device are known. Therefore, it is possible to set the overheat threshold to a suitable value according to the known characteristics. However, when the switching device is added as an external device to the IC, since characteristics of the switching device vary from product to product and cannot be identified, it is difficult to set the overheat threshold to a suitable value. For this reason, when the switching device is added as an external device to the IC, it is difficult to implement an overheat protection function.